You mean marking the logs? Numerous ways, you can start with just a tape measure and some chalk! they make commercial log markers as well, with wheels, etc and some guys here make a PVC pipe contraption that has a little slit at one end and bleeds marking chalk poweder.
Fastest is two ways..just develop an eye and get close enough If you or real actually practice and check it weith a tape or stick, youll get it pretty easy.
second is, use your limbing saw. I fell with a big saw, then immediately switch to a small saw. some have 14 inch bars, some 16. I use the bar length as a quick eyeball guide to back up my normal guesstimates. works fine, start at the butt end, make some little test cuts, walk it over that tip bar, do it again, etc. Walk it right down the trunk takes like 60 seconds tops to walk down a large log. But I only ever actually measure the big trunk with that method, or a tape measure and a mark, everything else is a fast eyeball. I start cutting out at the branches, without removing the branches from the tree, as much as possible, and work my way back down to larger diameter then larger saws..
I think the worst way is to quick get nuts and drop all the big branches before you cut the smalls off. I cringe when I see vids of guys doing that, just trashing all that good firewood. Makes a much bigger mess, more dangerous as you are fighting your way thyrough big branch crap on the ground where you want to walk, and you wind up wasting wood and/or having a lot of difficult cuts that could have been easy.
Same deal here, top and front loading, the top just swings up, so I can be off a little and get atter or longer pieces and it will still fit, although in practice, conicidently verified yesterday while cutting, my eyeball is pretty good, made some marks then ran the tape against them, half inch either way was the spread, and thats moving *fast*, close enough for personal firewood. I I was selling it I would get one of those mingo markers or make the plastic contraption, etc..
They make actual logging tape measures, but I don't have one.